A. Field of the Invention
The present invention relates generally to systems for collecting fluid samples and, more particularly, to an apparatus and method for collecting fugitive emissions from process equipment.
B. Description of the Related Art
Industrial plants that handle volatile organic compounds (VOCs) typically experience unwanted emissions of those compounds into the atmosphere from point sources such as smokestacks and non-point sources such as valves, pumps, and fittings installed on pipes and vessels containing the VOCs. Emissions from non-point sources, referred to as "fugitive" emissions, typically occur due to leakage of the VOCs from joints and seals. Fugitive emissions from control valves may occur as leakage through the packing between the valve stem and body of the valve. Valves employed in demanding service conditions involving frequent movement of the valve stem and large temperature fluctuations typically suffer accelerated deterioration of the valve stem packing, resulting in greater fugitive emissions than valves in less demanding service.
While improvements in valve stem packing materials and designs have reduced fugitive emissions and lengthened the life of valve packing, the monitoring of fugitive emissions has become important as a means to identify and reduce fugitive emissions and comply with new more stringent regulation of emissions. The Environmental Protection Agency (EPA) has promulgated regulations specifying the maximum permitted leakage of certain hazardous air pollutants from control valves, and requiring periodic surveys of emissions from control valves.
Current methods of monitoring fugitive emissions involve manual procedures using a portable organic vapor analyzer. This manual method is time consuming and expensive to perform, and can also yield inaccurate results due to ineffective collection of the fugitive emissions from the equipment being monitored. If measurements are made on a valve exposed to wind, emissions from the valve may be dissipated before the vapor analyzer can properly measure the concentration of the emissions. Also, if the analyzer is not carefully moved around the valve to capture all the emissions from the valve, an inaccurate measurement will result. Manual measurement methods also require plant personnel to dedicate a significant amount of time to making the measurements, distracting from their other duties.
Automated monitoring and detection of fugitive emissions can yield significant advantages over existing manual methods. The EPA regulations require surveys of fugitive emissions at periodic intervals. The length of the survey interval may be monthly, quarterly, semi-annual, or annual; the required surveys becoming less frequent if the facility operator can document fewer than a certain percentage of control valves with excessive leakage. Thus, achieving a low percentage of leaking valves reduces the number of surveys required per year. In a large industrial facility where the total number of survey points can range from 50,000 to 200,000 points, this can result in large cost savings. By installing automated fugitive emission sensing systems onto valves subject to the most demanding service conditions and thus most likely to develop leaks, compliance with the EPA regulations can be more readily achieved for the entire facility. This results in longer intervals between surveys for all of the valves, significantly reducing the time and expense of taking measurements manually from the valves without automated sensing systems.
Early detection of fugitive emissions from leaking valves also enables repairs to be made on a more timely basis, reducing the quantity of hazardous material emitted and reducing the cost of lost material. Accurate sensing of fugitive emissions provides an early warning system which can alert the facility operator to a potential valve seal failure and enable preventive measures to be taken before excessive leakage occurs.
However, employing an automated fugitive emission sensing system in an industrial environment requires designing a sample retrieval system which can efficiently collect fugitive emissions emanating from a piece of equipment and transport the emissions to gas sensors. The sample retrieval system must be capable of delivering a sample stream at a known flow rate in order to permit the gas sensors to make accurate and consistent measurements of the concentration of fugitive emissions. The sample retrieval system must be inexpensive to manufacture, and use a power source that is readily available in typical process plant, in order to keep installation costs to a minimum. The system must be suitable for use in hazardous areas subject to a risk of explosion, requiring electrical equipment to be of intrinsically safe or explosion-proof design. It also must be able to operate in harsh environments, including areas subject to hosing, high humidity, high and low temperatures, and vibration. The system also must be simple and reliable, in order to keep maintenance costs to a minimum.
Accordingly, it is an object of the present invention is to provide an apparatus and method for automatically collecting emissions from equipment that is suitable for industrial applications. Another object of the present invention is to provide an apparatus and method that provides for accurate and consistent collection of fugitive emissions. Another object of the present invention is to provide an apparatus to collect emissions can operate safely in hazardous environments. Another object of the present invention is to provide an apparatus and method to collect emissions that uses an existing pneumatic power source to collect the emissions. Yet another object of the present invention is to provide an apparatus that is simple and inexpensive to install. A further object of the present invention is to provide an apparatus and method to collect emissions that provides for low maintenance operation. A further object of the present invention is to provide an apparatus and method to collect emissions data and store the data for later retrieval. A further object of the present invention is to provide an apparatus and method to collect emissions data and communicate this data to a remote plant process control system. Yet a further object of the present invention is to provide an apparatus and method to collect emissions data and use this data to control the plant to reduce or eliminate the emissions. Yet a further object of the present invention is to provide an apparatus and method to collect emissions data and communicate this data to a remote plant process control system to enable control of the plant to reduce or eliminate the emissions.